The present invention relates to a solid electrolytic capacitor comprising a solid electrolyte layer and an electrically conducting layer (an electrically conducting layer comprising metallic powder or an electrically conducting layer comprising an electrically conducting carbon layer and a layer formed thereon and comprising metallic powder) in which at least one of said layers contains a rubber-like elastic material and a production process thereof. More specifically, the present invention relates to a solid electrolytic capacitor which can be made compact and can be endowed with high-capacitance and low-impedance and is excellent in external force-relaxing properties, productivity, heat resistance and moisture resistance, etc., and a production process thereof.
The present invention also relates to a solid electrolyte, a conducting paste comprising metallic powder and an electrically conducting carbon paste for use in a solid electrolytic capacitor.
In general, a solid electrolytic capacitor is formed through the following steps: a dielectric oxide film layer is formed on a positive electrode substrate formed of a metallic foil which undergoes etching treatment and has a large specific surface area; a solid semiconducting layer (hereinafter referred to as xe2x80x9csolid electrolyte layersxe2x80x9d) serving as a counter electrode is formed outside the oxide film layer; preferably a conducting layer comprising metallic powder or a conducting layer comprising a conductive carbon layer and a layer formed thereon comprising metallic powder is further formed on the outer side of the solid electrolyte layer; and a lead wire is connected thereto, thereby forming the basic elements of a capacitor. Subsequently, the entirety of the elements is completely sealed by use of an epoxy resin or the like. The thus-obtained product is widely used as a capacitor component in electric appliances.
In recent years, in order to meet requirements for digitization of electric apparatuses and increase in processing speed of personal computers, solid electrolytic capacitors are demanded to have small size, high capacitance, and low impedance in a high-frequency range.
In order to meet demands for such solid electrolytic capacitors, suggestions have been made with regard to solid electrolytes, conducting materials, etc.
For the solid electrolyte, it is heretofore known to use, for example, an inorganic semiconductor material such as manganese dioxide and lead dioxide, an organic semiconductor material such as TCNQ complex salt, an intrinsic electrically conducting polymer having an electric conductivity of from 10xe2x88x923 to 5xc3x97103 S/cm (JP-A-1-169914 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) corresponding to U.S. Pat. No. 4,803,596) or an electrically conducting polymer such as xcfx80-conjugated polyaniline (JP-A-61-239617), polypyrrole (JP-A-61-240625), polythiophene derivative (JP-A-2-15611, U.S. Pat. No. 4,910,645) or polyisothianaphthene (JP-A-62-118511).
Capacitors using manganese dioxide for the solid electrolyte are disadvantageous not only in that when manganese nitrate is thermally decomposed to form manganese dioxide, the oxide dielectric film once formed on the anode foil is ruptured, but also in that the impedance property is not satisfied.
In the case of using lead dioxide, cares on the environment are additionally required.
Capacitors using a TCNQ complex salt solid for the solid electrolyte have good heat molten workability and excellent electric conductivity but are considered to show poor reliability in the heat resistance at the solder joining (soldering heat resistance) because the TCNQ complex salt itself has a problem in the heat resistance.
Capacitors using an electrically conducting polymer for the solid electrolyte are free of rupture of dielectric film and favored with high impedance property but disadvantageously deficient in the heat resistance, thermal shock resistance and vibration resistance.
With respect to the method for forming a solid electrolyte using an electrically conducting polymer, for example, a method of melting an electrically conducting polymer (solid electrolyte) as described above on a dielectric film layer on the surface of a valve-acting metal having fine void structures to form an electrically conducting polymer layer, and a method of depositing the above-described electrically conducting polymer on the dielectric film layer are known.
More specifically, in the case of using, for example, a polymer of a 5-membered heterocyclic compound such as pyrrole or thiophene for the solid electrolyte, there are known a method of forming an electrically conducting polymer layer having a necessary thickness by repeating a series of operations of dipping an anode foil having formed thereon a dielectric film in a lower alcohol and/or water-based solution of a 5-membered heterocyclic compound monomer and after pulling it up, again dipping the foil in an aqueous solution having dissolved therein an oxidizing agent and an electrolyte to cause chemical polymerization of the monomer (JP-A-5-175082), a method of coating simultaneously or not simultaneously a 3,4-ethylenedioxythiophene monomer and an oxidizing agent each preferably in the form of a solution on the oxide film layer of a metal foil to form an electrically conducting polymer layer (JP-A-2-15611 (U.S. Pat. No. 4,910,645) and JP-A-10-32145 (European Patent Laid-Open Publication 820076)), and the like.
As the oxidizing agent for use in conventional techniques, for example, chemical polymerization of 5-membered heterocyclic compounds such as thiophene, there are known iron (III) chloride, Fe(ClO4)3, organic acid iron (III) salt, inorganic acid iron (III) salt, alkyl persulfate, ammonium persulfate (hereinafter simply referred to as xe2x80x9cAPSxe2x80x9d), hydrogen peroxide, K2Cr2O7, etc., (JP-A-2-15611), cupric compounds, silver compounds, etc., (JP-A-10-32145 (European Patent Laid-Open Publication 820076)).
In recent years, a method for producing a polyaniline composite is proposed, where powdered polyaniline is used as an electrically conducting starting material, rubber and/or thermoplastic resin is used as the matrix material and the powdered polyaniline is dispersed and compounded in the rubber and/or thermoplastic resin to form a polyaniline composite having mechanical strength and flexibility (JP-A-64-69662).
Furthermore, a method for producing a capacitor is proposed, where a composite film is formed on the metal oxide of a capacitor electrode from a polyaniline solution containing from 1 to 25 mass % of a polymer binder and an electrically conducting polymer layer comprising polyaniline having added thereto anion is further formed on the composite film (JP-A-5-3138).
According to the above-descried methods, it is necessary for forming an electrically conducting polymer layer to previously form a thin electrically conducting layer on the oxide film as an insulator by chemical polymerization. Furthermore, there are problems mentioned below in suitability applying these methods to respective capacitors.
First, in the case of electrolytic polymerization, if the polymer has poor flexibility, the increase in viscosity causes reduction in capacitance. More specifically, when an aluminum foil having formed thereon a dielectric material obtained by etching the surface is dipped with an oxidizing agent solution and then dried, an oxide film having high viscosity is formed on the surface of a porous body. As a result, microfine pore openings present on the surface of the porous body are clogged. Furthermore, a polymer is formed on the surface by the contact with a monomer and the polymer is not formed inside the pores, which causes reduction in capacitance.
Second, in the case of chemical polymerization, the amount of polymer adhered by one polymerization step is small, accordingly, the dipping must be repeated with predetermined number of steps. Thus, a method advantageous in view of productivity is demanded.
Third, close contact or good adhesive property between the dielectric film and the solid electrolyte is required. If the adhesive property is poor, the product deteriorates or the uniformity is lost in the production, as a result, the production yield decreases or the durability in use has a problem.
In order to solve these problems, the electrically conducting polymer such as polypyrrole is electrolytically or chemically polymerized and the polymer obtained is used for the solid electrolyte of a solid electrolytic capacitor in the above-described methods. However, capacitors obtained are not satisfied in the uniformity of the electrically conducting polymer layer and properties as an electrolytic capacitor such as soldering heat resistance and impedance properties are not satisfactory.
For the electrical conducting layer used to join the cathode lead terminal and the solid electrolyte layer, an electrically conducting paste comprising an electrically conducting filler and a synthetic resin binder is usually used. A metal powder such as gold, silver, copper, etc., and carbon powder are generally used for an electrically conducting filler. The synthetic resin usually used includes epoxy resin, phenol resin and the like. Besides these resins, polyamide or polyimide resin, fluororesin (JP-A-5-152171) and acrylic resin (JP-A-7-233298) are also known.
Conducting carbon pastes have been used as die-bonding materials serving as adhesives between a silicon chip and a lead frame, or in a conducting paste layer of a solid electrolytic capacitor. In addition, a conducting paste containing a fluorine-containing polymer serving as a binder resin is also proposed (JP-A-2-5304). Such conducting pastes for die-bonding are demanded to have high conductivity, high heat resistance, low contraction stress generated during die bonding, and low water absorption ratio after die bonding. In addition, during heating conjunction, the paste must have ability to reduce stress generated between a silicon chip and a lead frame.
However, silver pastes using a common synthetic resin for the binder are high in the modulus of elasticity and a high stress is often generated by the reflow and the like, which causes increase in the leakage current or heat deterioration of the impedance due to separation at the paste interface. Furthermore, this kind of paste has high water absorption, accordingly, the performance thereof is liable to deteriorate in high temperature and high humidity conditions.
Some silver pastes use fluororesin as the binder but these pastes have also high modulus of elasticity and a high stress is generated by the reflow or the like to cause defect.
Silver, which is excellent in cost and performance, is widely used as a conducting material. However, due to unfavorable migration phenomenon of silver, when used as a paste of a solid electrolytic capacitor, a conducting silver paste is often used only after an electrically conducting carbon paste is applied.
There are many proposals regarding a conducting material, a binder, and a solvent used in an electrically conducting carbon paste. For example, JP-A-9-31402 discloses combination of natural graphite (flake graphite having a size of 10-20 xcexcm) and carbon black, serving as a conducting material. JP-A-5-7078 discloses a carbon powder having projections and serving as a conducting material. JP-A-4-181607 discloses the combination of carbon black having a size of 20 xcexcm or less and a synthetic resin, serving as a combination of a conducting material and a binder. JP-A-7-262822 discloses the combination of a flake graphite powder and a micro-graphite powder (aspect ratio: 10 or more, average particle size: 10 xcexcm or less) and an epoxy resin, serving as a combination of a conducting material and a binder. JP-A-61-69853 discloses the combination of graphite and a fluorine-containing polymer (e.g., PTFE micro-particles), serving as a combination of a conducting material and a binder. JP-A-4-177802 discloses the combination of a carbon powder and a glycidyl ether, serving as a combination of a conducting material and a solvent. Furthermore, a number of synthetic resins serving as a binder, such as polyethylene, epoxy resins, and phenolic resins, are proposed.
However, an electrically conducting carbon paste produced from natural graphite has a disadvantage of low conductivity, since natural graphite has flake form and therefore attains poor packing, and contains a large amount of impurities. In addition, when the paste of this type is applied, peeling of the paste tends to occur at the interface thereof since the surface thereof has low irregularity, and the paste has a problem that the heat deterioration of impedance tends to occur.
Meanwhile, an electrically conducting carbon paste produced from carbon black contains very small particles, and thus enhanced packing cannot be obtained and conductivity of the paste is difficult to increase in the same way as the paste produced from natural graphite. These natural graphite-type and carbon black-type conducting carbon pastes must be subjected to dispersion treatment during paste preparation.
Employment of an epoxy resin serving as a binder provides some advantages, including low cost and easy handling. However, the epoxy resin has some drawbacks, including high rigidity, and low capacity of relaxation in response to reduction of stress generated between a chip and a lead frame during heating treatment such as reflow soldering in accordance with increase in chip size. In addition, the resin has high water-absorption ability, and thus deterioration of moisture resistance tends to occur.
The present invention relates to a solid electrolytic capacitor comprising a solid electrolyte layer, an electrically conducting layer comprising metallic powder and an electrically conducting carbon layer which is optionally formed between the solid electrolyte layer and the layer comprising metallic powder, in which at least one of said layers contains a rubber-like elastic material; a production process thereof; and solid electrolyte, an electrically conducting paste comprising metallic powder and an electrically conducting carbon paste, each of which will be described in detail below.
1) A solid electrolytic capacitor comprising a valve-acting metal having a dielectric film layer formed on the surface thereof, a solid electrolyte layer and an electrically conducting layer which are formed on the dielectric film layer, wherein at least one of said solid electrolyte layer and electrically conducting layer contains a rubber-like elastic material.
2) The solid electrolytic capacitor as described in the above 1), wherein the electrically conducting layer is an electrically conducting layer containing metallic powder or an electrically conducting layer comprising an electrically conducting carbon layer and a layer containing metallic powder formed on the conducting carbon layer.
3) The solid electrolytic capacitor as described in the above 1) or 2), wherein the rubber-like elastic material is contained in the solid electrolyte layer.
4) The solid electrolytic capacitor as described in the above 2), wherein the rubber-like elastic material is contained in the conducting carbon layer.
5) The solid electrolytic capacitor as described in the above 2), wherein the rubber-like elastic material is contained in the electrically conducting layer containing metallic powder.
6) The solid electrolytic capacitor as described in the above 2), wherein the rubber-like elastic material is contained in the solid electrolyte layer and the electrically conducting carbon layer.
7) The solid electrolytic capacitor as described in the above 2), wherein the rubber-like elastic material is contained in the solid electrolyte layer and the electrically conducting layer containing metallic powder.
8) The solid electrolytic capacitor as described in the above 2), wherein the rubber-like elastic material is contained in the electrically conducting carbon layer and the electrically conducting layer containing metallic powder.
9) The solid electrolytic capacitor as described in the above 2), wherein the rubber-like elastic material is contained in all of the solid electrolyte layer, the electrically conducting carbon layer and the electrically conducting layer containing metallic powder.
10) The solid electrolytic capacitor as described in any one of the above 1) to 9), wherein the solid electrolyte layer has a film-like or lamellar structure.
11) The solid electrolytic capacitor as described in the above 1), 3), 6), 7) or 9), wherein the solid electrolyte layer is formed of an electrically conducting polymer composition in the form of a film-like or lamellar structure containing from 0.01 to 25 mass % of a rubber-like elastic material.
12) The solid electrolytic capacitor as described in the above 11), wherein the rubber-like elastic material is at least one of natural rubbers and synthetic elastomers.
13) The solid electrolytic capacitor as described in the above 11) or 12), wherein the rubber-like elastic material is fluororubber.
14) The solid electrolytic capacitor as described in the above 11), wherein the electrically conducting polymer is a polymer containing at least one repeating unit of a divalent group selected from pyrrole, thiophene, aniline and derivatives thereof.
15) The solid electrolytic capacitor as described in the above 2), 5), 7), 8) or 9), wherein the electrically conducting layer containing metallic powder comprises an electrically conducting filler containing metal powder and a binder mainly comprising fluororubber.
16) The solid electrolytic capacitor as described in the above 15), wherein 80 mass % or more of the binder is fluororubber.
17) The solid electrolytic capacitor as described in the above 15), wherein 80 mass % or more of the electrically conducting filler is silver powder.
18) The solid electrolytic capacitor as described in the above 15) or 17), wherein the electrically conducting filler has an average particle size of from 1 to 10 xcexcm.
19) The solid electrolytic capacitor as described in the above 15), 17) or 18), wherein the electrically conducting filler content is from 50 to 95 mass % and the binder content is from 5 to 50 mass %.
20) The solid electrolytic capacitor as described in the above 15), wherein the electrically conducting layer containing metallic powder is formed of an electrically conducting paste comprising an electrically conducting filler, a binder and an organic solvent.
21) A solid electrolytic capacitor obtained by sealing a capacitor device comprising a valve-acting metal anode having formed on the surface thereof a dielectric film, a solid electrolyte layer and an electrically conducting layer, with an insulating resin exclusive of the exposed areas of the anode lead terminal and the cathode lead terminal, wherein the solid electrolyte layer is an electrically conducting polymer layer and the electrically conducting layer is formed of an electrically conducting layer containing metallic powder described in the above 15) or 20).
22) The solid electrolytic capacitor as described in the above 21), wherein the electrically conducting layer comprises an electrically conducting carbon layer formed on the electrically conducting polymer layer and an electrically conducting layer containing metallic powder described in any one of the above 15) to 20), which is formed on the conducting carbon layer.
23) The solid electrolytic capacitor as described in the above 21) or 22), wherein the electrically conducting polymer layer is formed of poly (3,4-ethylene-dioxythiophene).
24) The solid electrolytic capacitor as described in the above 2), 4), 6), 8), 9) or 22), wherein the electrically conducting carbon layer is formed of an electrically conducting carbon paste predominantly comprising a conducting material, a binder and a solvent, and the conducting material contains artificial graphite in an amount of 80 mass % or more, and the artificial graphite has a fixed carbon content of 97 mass % or more, has an average particle size of 1-13 xcexcm, an aspect ratio of 10 or less, and contains particles having a particle size of 32 xcexcm or more in an amount of 12 mass % or less.
25) The solid electrolytic capacitor as described in the above 24), wherein the binder is a material of rubber-like elasticity which is swellable or suspendable in a solvent.
26) The solid electrolytic capacitor as described in the above 25), wherein the material of rubber-like elasticity which is swellable or suspendable in a solvent is at least one species selected from the group consisting of isoprene rubber, butadiene rubber, styrene/butadiene rubber, nitrile rubber, butyl rubber, an ethylene/propylene copolymer, acrylate rubber, polysulfide rubber, a fluoropolymer, silicone rubber, and a thermoplastic elastomer.
27) The solid electrolytic capacitor as described in the above 24), wherein the conducting material accounts for 30-99 mass % and the binder accounts for 1-70 mass % of the entire solid content of the conducting carbon paste.
28) A process for producing a solid electrolytic capacitor comprising steps of forming a solid electrolyte layer and an electrically conducting layer on a dielectric film layer which has been formed on the surface of a valve-acting metal, which comprises covering the valve-acting metal having formed on the surface thereof a dielectric film with a solution containing a monomer of an electrically conducting polymer and a solution containing an oxidizing agent one after the other once or a plurality of times to form an electrically conducting polymer composition film on the dielectric film, a rubber-like elastic material being contained in at least one of the monomer-containing solution and the oxidizing agent-containing solution.
29) The process for producing a solid electrolytic capacitor as described in the above 28), wherein the electrically conducting polymer composition contains from 0.01 to 25 mass % of a rubber-like elastic material.
30) The process for producing a solid electrolytic capacitor as described in the above 28) or 29), wherein the rubber-like elastic material is fluororubber.
31) A process for producing a solid electrolytic capacitor comprising steps of forming a solid electrolyte layer and an electrically conducting layer on a dielectric film layer which has been formed on the surface of a valve-acting metal, wherein the electrically conducting layer is formed on the solid electrolyte by using an electrically conducting paste comprising an electrically conducting material, a binder of rubber-like elasticity and a solvent.
32) The process for producing a solid electrolytic capacitor as described in the above 31), wherein the electrically conducting layer is a layer formed by using an electrically conducting paste comprising an electrically conducting material consisting of metallic powder, a binder of rubber-like elasticity and a solvent.
33) The process for producing a solid electrolytic capacitor as described in the above 31), wherein the step of forming electrically conducting layer comprises forming an electrically conducting carbon layer by using an electrically conducting carbon paste comprising a conducting material, a binder of rubber-like elasticity and a solvent, and then forming an electrically conducting layer containing metallic powder.
34) The process for producing a solid electrolytic capacitor as described in the above 31), wherein the step of forming an electrically conducting layer comprises forming an electrically conducting carbon layer by using an electrically conducting carbon paste comprising a conducting material, a binder of rubber-like elasticity and a solvent, and then forming a film of an electrically conducting layer containing metallic powder by using an electrically conducting paste comprising a conducting material consisting of metallic powder, a binder of rubber-like elasticity and a solvent.
35) The process for producing a solid electrolytic capacitor as described in any one of the above 31) to 34), wherein the step of forming solid electrolyte layer comprises covering the dielectric film with a solution containing a monomer of an electrically conducting polymer and a solution containing an oxidizing agent one after the other once or a plurality of times to form an electrically conducting polymer composition, a rubber-like elastic material being contained in at least one of the monomer-containing solution and the oxidizing agent-containing solution.
36) The process for producing a solid electrolytic capacitor as described in any one of the above 31) to 35), wherein the solid electrolyte layer has a film-like or lamellar structure.
37) The process for producing a solid electrolytic capacitor as described in the above 36), wherein the thickness of the film or each of the layers in the lamellar structure of the solid electrolyte falls within a range of approximately 0.1 xcexcm to 0.3 xcexcm.
38) A solid electrolyte formed from an electrically conducting polymer composition containing from 0.01 to 25 mass % of a rubber-like elastic material into a film-like or lamellar structure.
39) The solid electrolyte as described in the above 38), wherein the rubber-like elastic material is at least one of natural rubbers and synthetic elastomers.
40) The solid electrolyte as described in the above 38) or 39), wherein the rubber-like elastic material is fluororubber.
41) The solid electrolyte as described in the above 38), wherein the electrically conducting polymer is a polymer containing at least one repeating unit of a divalent group selected from pyrrole, thiophene, aniline and derivatives thereof.
42) A process for producing an article having a solid electrolyte formed of an electrically conducting polymer composition in the form of a film-like or lamellar structure, which comprises coating an article to be provided with solid electrolyte formed of an electrically conducting polymer composition on the surface thereof with a solution containing a monomer of an electrically conducting polymer and a solution containing an oxidizing agent one after the other once or a plurality of times to form an electrically conducting polymer composition film, a rubber-like elastic material being contained in at least one of the monomer-containing solution and the oxidizing agent-containing solution.
43) The process for producing an article having a solid electrolyte as described in the above 42), wherein coating is effected by dipping, applying, spraying or spreading.
44) The process for producing an article having a solid electrolyte as described in the above 42), wherein the rubber-like elastic material is added to the monomer-containing solution and/or the oxidizing agent-containing solution in the form of solution or dispersion.
45) An electrically conducting paste for solid electrolytic capacitors comprising an electrically conducting filler containing metal powder and a binder mainly comprising fluororubber.
46) The electrically conducting paste as described in the above 45), wherein 80 mass % or more of the binder is fluororubber.
47) The electrically conducting paste as described in the above 45), wherein 80 mass % or more of the electrically conducting filler is silver powder.
48) The electrically conducting paste as described in the above 45) or 47), wherein the electrically conducting filler has an average particle size of from 1 to 10 xcexcm.
49) The electrically conducting paste as described in any one of the above 45), 47) or 48), wherein the electrically conducting filler content is from 50 to 95 mass % and the binder content is from 5 to 50 mass %.
50) The electrically conducting paste as described in any one of the above 45) to 49), which contains an organic solvent.
51) An electrically conducting carbon paste for solid electrolytic capacitors predominantly comprising an electrically conducting carbon material, a binder, and a solvent, wherein the electrically conducting carbon material contains artificial graphite in an amount of 80 mass % or more, and the artificial graphite has a fixed carbon content of 97 mass % or more, has an average particle size of 1-13 xcexcm, an aspect ratio of 10 or less, and contains particles having a particle size of 32 xcexcm or more in an amount of 12 mass % or less.
52) The electrically conducting carbon paste for solid electrolytic capacitors as described in the above 51), wherein the binder is a material of rubber-like elasticity which is swellable or suspendable in a solvent.
53) The electrically conducting carbon paste for solid electrolytic capacitors as described in the above 52), wherein the material of rubber-like elasticity is at least one species selected from the group consisting of isoprene rubber, butadiene rubber, styrene/butadiene rubber, nitrile rubber, butyl rubber, an ethylene/propylene copolymer, acrylate rubber, polysulfide rubber, a fluoropolymer, silicone rubber, and a thermoplastic elastomer.
54) The electrically conducting carbon paste for solid electrolytic capacitors as described in any one of the above 51) to 53), wherein the conducting material accounts for 30-99 mass % and the binder accounts for 1-70 mass % of the entire solid content of the conducting carbon paste.